According to a radiator for a vehicle disclosed in JP-A-6-159986, dents or projections (dimples) are formed on inside walls of tubes at least at portions other than longitudinal ends (header insertion portions) of the tubes. This improves a coefficient of heat transfer between fluid flowing through the tubes (e.g. cooling water and hot water) and the tubes. Further, this restricts gaps between the header insertion portions of the tube ends and insertion holes formed on header tanks from excessively increasing, thereby preventing defective brazing between the tubes and the header tanks.
With respect to a method of manufacturing the tube, a work in a form of band plate is pressed so that dimples are formed. Further, to deal with variation of length of the tubes, it is proposed to control the timing of feeding the work into a pressing device and the timing of operation of the pressing device so that portions where the dimples are formed can be changed. After the dimples are formed, the work is shaped into a tube by a shaping roller and thereafter cut into a predetermined length. In this method, however, the work is fed intermittently. Therefore, it is difficult to continuously form the dimples, resulting in low productivity and production rate.
On the other hand, it is proposed to use a roller shaping machine having a roller formed with dents or projections on its outer peripheral surfaces in order to continuously form dimples. The roller has a portion having the dents or projections for forming the dimples and a portion without having the dents or projections. By this, portions where the dimples are formed and portions where the dimples are not formed, which corresponds the header insertion portions, are formed at predetermined intervals. In this method, however, the roller needs to be exchanged with another roller whenever the length of the tubes is changed. Therefore, it is difficult to deal speedily with variations in the length of the tubes.